1. Field of the Invention
The present invention relates to a heat-insulating, carbon fiber felting material excellent in heat insulating properties, particularly in a high temperature range.
More particularly, the present invention relates to a carbon fiber felting material which exhibits excellent heat resistance and morphological stability within the temperature range of 500.degree. to 2,800.degree. C. as well as excellent heat insulating properties against radiant heat transfer in particular.
Still more particularly, the present invention relates to a carbon fiber felting material so excellent in heat insulating properties within the high temperature range that the felting material can be used for heat insulation of high temperature furnaces and the like which are used in fusion of glass, firing of pottery, smelting of metals, sintering of ceramics, or heat treatment of carbonaceous materials.
The present invention further relates to a carbon fiber felting material so excellent in stability against thermal radiation that the felting material can be used as a heat insulating material with excellent performance in nuclear furnaces and nuclear power generating installations.
2. Related Art
Porous ceramic materials have heretofore been mostly used as heat insulating materials serviceable within a high temperature range. These heat-insulating ceramic materials have an excellent high-temperature stability. In order to lower the thermal conductivity, however, they are required to have a considerable porosity.
The pores of the porous ceramic materials are not of completely closed-cell type, but usually is considerably restrictive on gaseous flow therethrough. This is so, from the viewpoint of strength, because those pores are formed in such a way as to communicate with the outsides of the porous ceramic materials only through considerably small passages. This will be understandable if consideration is given to the fact that any shaped ceramic article is decreased in strength in the case where it includes so large defects around the peripheries of its pores as to allow a gas to easily flow therethrough.
Because of such morphological characteristics, the conventional heat-insulating ceramic materials are generally so weak against rapid cooling as well as rapid heating that they may involve a problem of frequent structural collapse beginning with the surfaces thereof upon changes of the temperature, which is called "spalling." In order to provide a heat-insulating ceramic material hardly subject to spalling, it is generally necessary to select a ceramic material low in porosity and hence poor in heat insulating properties, which must, therefore, be used in large amount.
As a solution to the foregoing problems, fibrous ceramic materials have heretofore been widely used as heat insulating materials. The fibrous ceramic materials exhibit an excellent heat insulating effect. However, they are generally expensive due to a difficulty encountered in production thereof. This is one reason for the high price of a high-temperature furnace.
On the other hand, the predominant mode of heat transfer shifts to radiant heat transfer with relatively decreasing contribution of convective as well as conductive heat transfer when the temperature reaches the high-temperature range of at least 500.degree. C. This presents a problem that, when the performance of a heat insulating material is considered in association with an aspect of its heat insulation mechanism, the heat insulating material effective in the low-temperature range of at most 200.degree. C. does not necessarily exhibit good performance in the high-temperature range.
Particularly, heat-insulating fibrous ceramic materials exhibit an excellent heat insulating effect in the low-temperature region, but are so poor in capability of radiation absorption and scattering as to provide an insufficient heat insulating effect against radiant heat transfer in the high-temperature range, because of their generally high transparency and very high surface smoothness characteristic of such fibers.
On the other hand, either carbonaceous or graphitic materials, e.g., mesophase pitch type materials in particular have heretofore attracted attention of little significance as heat insulating materials because they are generally high in thermal conductivity to allow for large conductive heat transfer therethrough.
Since these materials are high in absorbance for radiations ranging from ultraviolet radiation to infrared radiation within a wide wave range and endowed with high morphological stability at high temperatures, however, it has been believed that they could probably be used as heat insulating materials if they were provided with a morphology highly capable of radiation scattering.
An object of the present invention is to provide a heat insulating material which can solve not only the problem that the conventional heat insulating materials for use in the high-temperature range are weak against rapid temperature changes and generally insufficient in the heat insulating effect against radiant heat transfer, but also the problem that the heat-insulating, fibrous ceramic materials are generally expensive and insufficient in the heat insulating effect against radiant heat transfer.